they have your $100 in deposits on their books, but in reatliy have only $5-$10 in actual cash of that deposit since the rest has been lend out. When people lose confidence in the system they may chose to demand their deposits back, but if you now want to withdraw the $100 you deposited the bank will only have $10 to give you which means the bank is insolvent. In order to protect banks from bank runs governments created deposit insurance so that depositors like you and me know that our deposits are guaranteed up to a certain amount so that we do not have to worry about it and so that private banks can continue to create money out of thin air and charge us interest on it. I hope this answers your question. Please continue to visit the Capital Research Institute as we unravel the web of international finance. Thank you.